Tour Guides for Athens

Hi Jan! How many people? It affects per-person cost. Also, be aware that many or "taxi/van tours" offer a driver, but NOT a "licensed guide."(latter involves years of classics education + tough tests). A "driver guide" cannot accompany you into an ancient site or museum, so u will be on your own, with a brochure he gives you. PLUS, many of major sites (Acropolis, Roman Agora, etc)are on no-car streets, so u wd be paying for a vehicle that lets u off some distance away.
All the above is just to warn you off of highly-promoted "tours" that essentially offer just over-priced chauffeurs. There ARE good resources that are affordable especially if u have a group This site gives you excellent LICENSED tour guides with/without vehicles as u wish; you can read & select; all state their credentials in detail & have wonderful reviews: http://tourguides.viator.com/Listing.aspx?Country=Greece&Region=Attiki&City=Athens

I recently took 7 people to Greece and we used the Athens Free Walking Tour group. It was low cost (tip 5 euro per person, we overtipped it was so good) and gives a good overview of Athens and it's ancient and modern history. http://www.athensfreewalkingtour.com/index.php